1. Field of the Invention
The present invention is directed to apparatus and a method for automatically measuring the blood pressure of an individual and specifically to apparatus and a method for processing a pressure signal of an automatic blood pressure gauge which pressure signal includes a blood-pressure pulse signal component.
2. Description of the Prior Art
A conventional automatic blood pressure gauge includes a resilient inflatable cuff and an electric pump. The pump is controlled by a microprocessor to inflate the cuff with a fluid, such as air, to a preset pressure. In addition, this automatic gauge includes a pressure transducer that measures the instantaneous air pressure levels in the cuff. The pressure signal produced by the transducer is used to determine both the instantaneous air pressure of the cuff and the blood pressure pulse of the individual. This pressure signal is generally band-pass filtered, digitized and processed by the microprocessor to produce values representing the mean, systolic and diastolic blood pressure measurements of the individual.
In operation, the cuff is affixed to the upper arm area (or other extremity) of the patient and is then inflated to a pressure greater than the suspected systolic pressure, for example, 150 to 200 millimeters of mercury (mmHg). This pressure level collapses the main artery in the arm, effectively stopping any blood flow to the lower arm. Next, the cuff is deflated slowly and the signal provided by the pressure transducer is monitored to detect cuff pressure variations caused by the patient's blood pressure pulse, which is mechanically coupled to the cuff.
In general, the pulse component of the pressure signal has a relatively low amplitude, on the order of one percent of the total signal. A low-level detected blood pressure signal first appears when the cuff pressure is released to a level which allows some blood flow into the collapsed artery. As cuff deflation continues, the blood-pressure pulse signal rises in amplitude as more of the collapsed artery is allowed to expand in response to the pumping action of the heart. At some point, however, the pulse signal reaches a maximum amplitude level and then begins to decrease. This reduction in amplitude occurs as the artery becomes more fully open, the pumped blood flows without significantly expanding the artery, and the degree of mechanical coupling between the cuff and the arm of the patient is reduced.
In many automatic blood pressure measuring systems, the systolic and diastolic pressures are determined based on the cuff pressure at which the blood-pressure pulse signal exhibits maximum amplitude. Such a system is described in U.S. Pat. No. 4,735,213 entitled DEVICE AND METHOD FOR DETERMINING SYSTOLIC BLOOD PRESSURE, which is hereby incorporated by reference for its teaching on automatic blood pressure gauges. In this system, the diastolic blood pressure is determined as the cuff pressure, after the maximum pulse amplitude has been measured, at which the amplitude of the pulse signal is 70% of its maximum value.
Another exemplary system is described in U.S. Pat. No. 4,949,710 entitled METHOD OF ARTIFACT REJECTION FOR NONINVASIVE BLOOD-PRESSURE MEASUREMENT BY PREDICTION AND ADJUSTMENT OF BLOOD-PRESSURE DATA, which is hereby incorporated by reference for its teaching on automatic blood pressure gauges. In this system, the systolic and diastolic blood pressure levels are determined as the respective cuff pressures corresponding to the amplitude of the blood-pressure pulse signal being 60% of the maximum value, prior to reaching the maximum value; and 80% of the maximum value, after reaching the maximum value.
FIG. 1a is a plot of the pressure signal versus time for a conventional automatic blood pressure gauge. This exemplary signal is generated by the cuff being quickly inflated to a preset initial pressure, greater than the systolic pressure, linearly deflated to a pressure below the diastolic pressure and then quickly deflated the rest of the way. The blood-pressure pulse signal is shown as a waveform superimposed on the linear deflation portion of the pressure curve. For clarity, the relative amplitude of this pulse signal is exaggerated in FIG. 1a.
FIG. 1b is a plot of the blood-pressure pulse signal shown in FIG. 1a, separated from the linearly decreasing pressure signal. FIG. 1c is a plot of the peak amplitude of the signal shown in FIG. 1b. As illustrated by FIG. 1c, the amplitude of the pulse signal increases gradually until a time S, at which the linearly decreasing cuff pressure is the same as the systolic pressure of the patient. The amplitude of the pulse signal then increases at a greater rate from time S to time M at which the maximum amplitude is reached. The blood pressure level corresponding to this maximum pulse amplitude is commonly referred to as the mean arterial pressure (MAP). From this maximum amplitude, the pulse signal decreases rapidly to a time D, at which the cuff pressure is the diastolic pressure. The signal amplitude decreases from the point D until the cuff is entirely deflated.
In order to accurately determine the systolic and diastolic pressures of the patient, it is important to ensure a uniform sampling density for the points which define the curve 1b. This is achieved, in part, by ensuring that the deflation curve shown in FIG. 1a is substantially linear.
The deflation of a fixed volume container through a fixed orifice area generates a pressure deflation curve which approximates a decaying exponential. One method to obtain a linear deflation rate is to use a valve having a controllable orifice area, for example, a needle valve which can be mechanically actuated to change its orifice area. Valves of this type, however, can be difficult to control.
The size of the valve orifice may be controlled using a closed loop control system, which changes the orifice area of the valve in a manner which holds the first derivative of the measured cuff pressure value substantially constant. To minimize errors and to ensure a short settling time from pressure transients caused, for example, by patient motion, it is desirable to use a control loop having a relatively short time constant.
This type of system, however, may affect the measurement of the blood pressure pulses. It may interpret the blood-pressure pulse signal as a transient pressure change and attempt to compensate for it in order to maintain a constant deflation rate. This action may undesirably reduce the amplitude of some of the pulses, thus changing the shape of the pulse amplitude curve shown in FIG. 1c.
Another problem may arise when the cuff deflation system uses switched solenoid valves to obtain a desired deflation rate. As these valves are switched on and off, a rippling pressure signal is generated which may be detected by the pressure transducer of the blood pressure gauge.
In order to be cost effective, the valve switching frequency should be kept as low as possible. When this is done, however, signal components resulting from the switching signal may contaminate the blood-pressure pulse signal. These components are usually removed using a low-pass filter having a cut-off frequency between the highest pulse frequency which may be of interest and the switching frequency.
The frequency spectrum of the blood-pressure pulse signal, however, may have significant components which occupy frequency bands between the cut-off frequency of the low-pass filter and the valve switching frequency. These components of the pulse signal may be undesirably attenuated by the low-pass filter which removes the switching signal, distorting the pulse waveform. This distortion may result in erroneous pulse amplitude measurements.
In conventional systems, this problem is solved either by using some other type of deflation valve or by selecting a valve switching frequency that is well above the band of frequencies which may be occupied by the blood-pressure pulse signal. This selection may undesirably reduce the range of effective flow rates that the valve may provide or it may increase the cost of the device if a high performance solenoid valve is needed to operate at the desired switching frequency.
A related problem occurs when the pressure signal is a sampled data signal. Even when the frequency of the valve switching signal is much greater than the frequency of the blood-pressure pulse signal, the sampling system may introduce an artifact of the switching signal into the detected pressure signal. This artifact occurs when the switching signal is translated into a frequency in the same band of frequencies as the blood-pressure pulse signal due to aliasing distortion of the sampled data signal.